If you are travelling outside the peak season it's usually easier and cheaper to book direct with one of the mobile home companies (eurocamp, canvas, vacancesoleil etc). Prices are similar and your dates are guaranteed, and you get to choose your campsite and style of mobile home. Rather than take potluck.
Ferry discounts - yes, flight discounts - no. Try moneysavingexpert's flight checker for the best deal, or book early.